This invention relates to a magnetic brush type developing device for use in an electrophotographic copying machine, and more particularly a magnetic brush type developing device used with a one component developer as is per se well known.
In an electrophotographic copying machine using a magnetic brush developing device, copied matter is obtained by reflecting light from an original to be copied, projecting the light onto a uniformly charged photosensitive member to form thereon an electrostatic latent image corresponding to the original, developing the latent image by a developer to render it visible, and then fixing the latent image on the member or after it is transferred onto a transferring paper such as plain paper. Developer (not shown) is stored in a developing chamber in which a cylindrical sleeve made of nonmagnetic material is so arranged that is is adjacent to the surface of the photosensitive member and a part of the sleeve is dipped into the developer. Inside of this sleeve, a plurality of magnets are arranged adjacent to the inside surface of the sleeve and with a given spacing, and the sleeve and the magnets are so constituted as to be able to relatively rotate. Accordingly, developer in the developer chamber is transported along the sleeve surface with the rotation of the sleeve or the magnets, and a visible image is formed on a photosensitive member as the developer comes into contact with the surface of the photosensitive member. When the developer is transported from the developer chamber, it is regulated at a certain fixed height on the sleeve by a regulating plate arranged adjacent to the outside surface of the sleeve. In other words, the developer adhered to the sleeve surface is transported to a developing position in the form of brush like tufts. Such a developing device as mentioned above is known as a magnetic brush type developing device.
Most of the presently popular electrophotographic copying machines are using a developer comprising magnetic powder (the so-called carrier) and fine grains of colored resin (the so-called toner) which, being constituted independently, define a type of developer called two-component developer. In conradistinction to two-component developer, a developer comprising toner only--that is, so-called one or single-component developer--has been developed. Because this latter type developer comprises magnetized toner having a magnetic substance in resin and containing colorant as occasion demands, density deviation of the copied image due to gradual toner consumption (as caused when using a two-component developer) is not a problem. As a consequence, copying machines using one-component developer do not require any density detecting device, or special toner replenishing device and carrier changes, and have many advantages as of making the developing device itself compact and so forth. Owing to the above facts, single component developer is being gradually put into practical use.
Two-component developer normally comprises carrier comprising a magnetic substance of about 50-200.mu. in diameter of a large grain and fine grain toner of about 5-20.mu. in diameter of a grain, and iron powder is used as the magnetic substance for the carrier. The mixing ratio of carrier and toner is approximately 2-10% by weight of toner with respect to the iron powder.
In two-component developer, a carrier of iron powder is adhered onto the sleeve by influence of magnetic force generated by a plurality of magnets fixedly provided within the sleeve, and transported in accordance with the rotation of the sleeve. The toner is transported in a body with the carrier, because the toner is strongly adhered electrostatically around the surface of the carrier of iron powder; thus, it is very seldom that toner is scattered in the course of transportation. And, since the carrier particle is heavy and its diameter is large, it is rare that it runs out through a narrow opening and so forth.
On the other hand, with a single component developer there is no grain of large diameter corresponding to carrier in the two-component developer; moreover, there are quantities of resin components in the particles of developer, and components of magnetic substances comprising at most 20-80% by weight of the developer, which is far less than that of a two-component developer.
Where a single component developer as stated above is used in a magnetic brush type developing device, and the sleeve or the group of magnets is rotated at a reasonable rate of revolution (e.g. 300 rpm), developer is blown up and scattered in a haze and contaminates in and outside of the developer chamber, or the developer penetrates into rotating shaft bearings of the sleeve or the group of magnets and thereby increases the friction of rotation and subsequently presents an obstacle to rotation of the shaft. The adhesion force of single component developer to the sleeve is reasonably weak, because the ratio of the magnetic component contained in the developer is low. Due to the above, adhesion of the developer to a magnetized portion of the developing device cannot be obtained to the same degree as with a two-component developer, despite compensation made by increasing the magnetic force of the group of magnets. Therefore, the thickness of the layer of developer which is formed on the sleeve becomes thin to a degree of 1-1.5 mm and consequently the clearance between a regulating plate for forming brush-like tufts of the developer and the sleeve necessarily becomes reasonably narrow. The developer, transported through the narrow clearance which regulates its height, spreads towards the outside and swells at both ends of the regulating plate, because of its weak adhesion force to the sleeve; this affects the results of subsequent development.
And, as opposed to the case of toner replenishment of two-component developer, a replenishing quantity of single-component developer does not exert a direct influence upon image density. It is a distinct advantage of single-component developer that a user can decide on the addition of a replenishing quantity at his option; but if replenishment is effected to an extent that as a rotating shaft bearing of the sleeve or the group of magnets is immersed into the developer, it is feared that particles will penetrate into a bearing portion and increase the load or friction of rotation and then cause an obstacle to free movement of the bearing.
At present, several electrophotographic copying machines having a magnetic brush type developing device and using a single component developer have been proposed (e.g. Laid Open Patent Exposure Nos. 52-10146 and 52-34742 in Japan), most of which are of the type in which developer is added or supplied in only a necessary quantity from a toner storing portion equipped atop the sleeve and into the developer chamber of the developing device. However no means to solve the problems peculiar to the above mentioned single-component developer have been introduced.
The present invention is directed to solving the peculiar faults mentioned above that arise when using a single component developer (also sometimes called a magnetic toner) for a magnetic brush type developing device. In accordance with the invention, a partition plate is arranged adjacent to the surface of both axially-directed ends of the sleeve of a developing device; preferably, the partition plate is constructed having a thickness of 0.15-1.0 mm and a clearance between it and the sleeve surface is kept to less than 0.35 mm. And, where the partition plate is set in non-contact with the sleeve surface, an auxiliary partition member is arranged to contact with the partition plate, and a clearance between the auxiliary partition member and the sleeve surface is preferably kept to less than 0.35 mm.
The following is a detailed explanation of the present invention referring to the drawings attached hereto.